1. Field of the Invention
The present invention relates to a radiography system consisting of a radiation projector for projecting radioactive rays toward a subject and a radiographic imaging device for acquiring an image of the subject from the radioactive rays that are incident on the imaging device after penetrating the subject. The present invention relates also to a source controller for controlling a radiation source included in the radiation projector.
2. Description of the Related Art
In the medical field, radiography systems utilizing radioactive rays, such as x-rays, for imaging are widely known. An x-ray radiography system, an example of radiography systems, includes an x-ray projector for projecting x-rays toward a subject and an x-ray imaging apparatus for acquiring a radiograph or x-ray image of the subject from the x-rays that have penetrated the subject. The x-ray projector includes an x-ray source, a source controller unit, and an activation switch for inputting an x-ray radiation starting signal. The x-ray imaging apparatus includes an x-ray image detector for detecting an x-ray image or x-ray images from incident x-rays, and a console for controlling operation of the x-ray image detector and processing the x-ray images for various image-renderings.
In the field of x-ray radiography system, x-ray image detectors using a flat panel detector (FPD) in place of conventional x-ray film or imaging plates (IP) have recently been widely spread. The FPD has a large number of pixels arranged in a matrix to accumulate signal charges corresponding to x-rays incident on the respective pixels. The FPD converts the accumulated signal charges to a voltage signal through a signal processing circuit, to detect an x-ray image representative of graphic information on the subject, and output the detected x-ray image as digital image data.
Portable x-ray image detectors, called electronic cassettes, have also been used in practice, each of which contains a flat panel detector in a rectangular box-shaped body. The plane size of the electronic cassette is about the same as those of radiographic film cassettes and IP cassettes, so that the electronic cassette can be removably mounted in a conventional radiographic stand or table which is adapted to the film cassette or IP cassette. In addition, the electronic cassette can be used independently. For example, in order to image such a site of a test subject that is hard to image using a stationary image detector that is fixedly mounted in the stationary radiographic stand or table, the portable electronic cassette may be put on a bed with the test subject or may be held directly by the test subject. Moreover, the electronic cassette may be carried around for use in home medical care or emergency medical care at accident sites or disaster sites outside the hospital.
In the FPD type image detector, charge-resetting operation for clearing the accumulated charges off the pixels is periodically carried out before starting charge accumulating operation, in order to reduce the influence of noises to the minimum. Accordingly, it is generally necessary for the radiography system using the FPD to synchronize the timing of x-ray radiation with the end of charge-resetting operation and the start of charge accumulating operation. For this purpose, in an example, a wired interface is interconnected between the source controller unit and the x-ray image detector (electronic cassette) so that the source controller unit sends a signal as a cue to the electronic cassette at the start of x-ray radiation, upon which the electronic cassette proceeds to the accumulating operation.
As an automatic exposure control (AEC) system, a conventional x-ray radiography system is provided with a sensor, such as an ion chamber, separately from the electronic cassette. The sensor measures x-ray dose applied to the test subject, in order to stop x-ray radiation from the x-ray source when the integrated amount of x-ray dose gets to a predetermined threshold level.
There have also been suggested such AEC systems as disclosed in JPA 2003-302716, wherein a photo timer or AEC sensor is integrated into an FPD type electronic cassette instead of a separate sensor like an ion chamber. An output terminal of the photo timer is connected to an interruption signal input terminal of a radiographic stand or table, through which the photo timer is connected to an x-ray projector. An output signal from the photo timer may be an x-ray interruption signal (radiation stopping signal) or an analog signal (detection signal or voltage level). In the former case, electric charges from the photo timer are integrated inside the electronic cassette, and the integrated value is compared with a threshold level, so that the x-ray interruption signal will be output when the integrated value gets over the threshold level. In the latter case, the analog signal from the photo timer is integrated after being received on the x-ray projector and the integrated value is compared with the threshold level to decide the time to stop x-ray radiation.
Because a delay in radiation stopping procedure by the source controller will lower the quality of acquired x-ray image and overexpose the patient as a test subject to x-rays, the radiation stopping procedure should be done in no time. For instance, requisite exposure time for radiography of chest is 50 ms or so. Within such a short time, the radiography system must deactivate the x-ray source as soon as it is confirmed by the detection signal from the AEC sensor of the electronic cassette that the radiation dose reaches a sufficient amount. However, in a case where the source controller and the x-ray image detector using the FPD are connected to each other through a wired interface, the single interface must manage to exchange not only AEC signals for automatic exposure control, like the x-ray interruption signal as mentioned in the above prior art, but also a variety of other signals, such as a radiation starting signal, data of image acquisition settings. In that case, the signals are more likely to interfere with each other, hindering exact transmission of the AEC signals and increasing the risk of delay in the radiation stopping procedure.
As a solution for the above problem, it may be possible to control signal transmissions such that the x-ray image detector will not send other signals than those used for automatic exposure control while the AEC signals for automatic exposure control are exchanged between the source controller and the x-ray image detector. However, this solution will require complicated control of the signal transmissions.
Besides the above problem, another problem occurs when an electronic cassette with an AEC sensor attached should be used in combination with an already installed x-ray projector, which has its own AEC sensor, e.g. a conventional ion chamber. In that case, it is necessary to connect the AEC sensor of the electronic cassette to a source controller of the existing x-ray projector. In many cases, however, x-ray projectors and x-ray imaging apparatuses are manufactured by different markers from each other. When an x-ray projector and an x-ray imaging apparatus of different markers constitute a radiography system, it is hard to specify how the signals exchanged between these machines would be processed inside the respective machines. For this reason, it has been difficult to assure the quality of such an x-ray radiography system that consists of an x-ray projector and an electronic cassette of a different marker from the x-ray projector, and an AEC sensor of the electronic cassette is connected to a source controller of the x-ray projector. Even while the respective markers guarantee the individual machines, if these machines are combined into a radiography system, any of these machines may operate unexpectedly. Consequently, it has been difficult to guarantee that the radiography system consisting of machines of different markers will satisfy the requirement for the above-mentioned extremely high-speed processing to terminate the radiation.
The above prior art does not disclose any solution for ensuring precise and high-speed procedures for stopping radiation in time.